This invention relates to a process for the devolatilisation of polymers, more particularly to an improved process for the removal of volatile impurities from thermoplastic polymers.
In the manufacture of a wide variety of polymers it is common for them to contain impurities which are unwanted in final products made from the polymers. Such impurities typically include residual monomer, solvents that may be used in the preparation of polymer, and low molecular weight organic species such as dimers and trimers that may be formed during the polymerization process.
An important field to which the present invention is particularly, though not exclusively directed is the preparation of polymers and copolymers of styrene made by a continuous mass polymerization process, in which it is desirable to produce polymer products having a residual styrene monomer content below 150 ppm and whose content of oligomeric species, e.g. styrene dimer is also minimized. Such products are useful for the manufacture of food packaging where migration of residual monomer from the polymer into the food can cause problems of undesirable odor, and/or flavor (taint).
In known devolatilisation processes a molten polymer is admixed with a small amount of an inert volatile substance known as a stripping agent. Known stripping agents include water, methanol and carbon dioxide. The resultant mixture is pumped through a distributor pipe into a heated vessel held at a reduced pressure, which causes the added volatile substance to produce a large population of bubbles in the molten polymer mass. This promotes diffusion of volatile impurities out of the molten polymer and this is enhanced by the high surface area of the resultant foaming mass. An example of an arrangement for carrying out the above devolatilisation process is disclosed in the article by T. O. Craig entitled xe2x80x9cApplication of an Enhanced Flash-Tank Devolatilisation System to a Degassing Extruderxe2x80x9d, Advances in Polymer Technology, Volume 10, No. 4 (1990), pages 323 to 325.
Other known polymer devolatilisation processes are disclosed for example in U.S. Pat. No. 5,380,822 and published European Patent Application EP-A-0583082.
The admixing of the stripping agent into the molten polymer can be carried out in an extruder, or more preferably in a static mixer. The objective is to dispersively mix the stripping agent finely in the molten polymer upstream of the reduced pressure vessel (known as the devolatiliser). Such mixing is a difficult task, because the viscosity of the stripping agent may typically be a millionth of the viscosity of the molten polymer mass, and in addition sufficient pressure must be maintained during the mixing step to ensure that premature vaporization of the stripping agent does not occur. Such premature vaporization leads to slugs of stripping agent vapor in the mass and reduces the efficiency of the devolatilisation process.
The use of water as a stripping agent for various polymers is widely practiced in the art. It is known that the effectiveness of admixed water as a stripping agent does not always increase with the amount of water used. When the admixed water is added at a level that is more than 20 times (by weight of the total composition) the level of residual styrene present, its effectiveness diminishes markedly. The effect is attributed to the cooling and concomitant increase in mass viscosity caused by flashing of the water from an initial dispersed liquid droplet form to a vapor. This is discussed in the paper by Darribere, Streiff and Juvet, xe2x80x9cStatic Devolatilisation Plantsxe2x80x9d, presented at the 6th International Workshop on Polymer Reaction Engineering, Berlin, 1998.
The use of carbon dioxide in its supercritical state as a stripping agent in polymer devolatilisation is also known, as disclosed for example in published European Patent Application EP-A-0798314. Carbon dioxide is particularly attractive as a stripping agent since under supercritical conditions it is partially soluble in molten polystyrene (and other polymers) and this solubility enhances the effect of carbon dioxide (CO2) on the removal of impurities from the polymer. However, the admixing of carbon dioxide into the molten polymer requires very high pressures and intensive mixing to prevent formation of vapor slugs and the attendant disadvantages described below.
Furthermore, because of its extremely low viscosity at elevated temperatures and pressures, the creation of a fine dispersion of carbon dioxide in a molten polymer is difficult. EP-A-0798314 teaches that the amount of carbon dioxide admixed into the polymer must exceed the amount of residual monomer present for the process to work satisfactorily, and that addition of a nucleating agent to the polymer is particularly preferred. Such nucleating agents may however adversely affect the properties of the polymer, especially where clarity is important.
A further disadvantage in using carbon dioxide as a stripping agent comes from the fact that it is a non-condensable gas and therefore places an additional volumetric pumping duty on the vacuum source that is used to maintain reduced pressure in the devolatiliser vessel. This means that the vacuum pumps need to be larger and more expensive than would be the case if water were used as the stripping agent.
The present invention seeks to ameliorate the disadvantages of the known polymer devolatilisation techniques discussed above, whilst combining the advantages of using water and carbon dioxide as stripping agents in polymer devolatilisation.